Fixed RCODE-processing.

Extended RCODE is one 12-bit number. Until now it was not only
handled separately for old and new RCODEs, but it was also
interpreted wrong.

Unified RCODE processing - it's represented by only one number and
writing into the packet takes place at the end of query processing.

refs #300
ext-rcode: Fixed RCODE setting in error-processing
ext-rcode: Fixed error processing.
Merge branch 'master' into ext-rcode

Conflicts:
	src/knot/nameserver/process_query.c
namedb: support more flags / operations
namedb: interactive iterator (lmdb) + updated tests

Not supported in trie because of the API incompatibility.
namedb: cleanup, comments
namedb: implemented GEQ/LEQ for lmdb backend + tests
ext-rcode: Review fixes
ext-rcode: Minor fixes to the previous fixes.
ext-rcode: Missing include
ext-rcode: Fixed wrong assert.

Some RCODEs other than NOERROR may result in DNSSEC processing,
e.g. NXDOMAIN.
Merge branch 'master' into namedb_iter_seek
namedb: implemented clear() operation
Merge branch 'master' into ext-rcode
Merge branch 'namedb_iter_seek' into 'master'

Namedb iter seek

This merge request extends the namedb api for following operations:
- `clear()` which equals to deletion of all entries in the database
- `iter_seek()` which returns an iterator to given key (or first, last, leq, geq)
  - this is not supported in the trie yet, but it works for lmdb backend
The unittests are extended to cover these operations.

See merge request !302
review: remove nonsensical assert
review: more code, less fairy tails
Merge branch 'ext-rcode' into 'master'

closes #300
Merge branch 'master' into public-libs-common

Conflicts:
	src/libknot/internal/namedb/namedb.h
	src/libknot/internal/namedb/namedb_lmdb.c
	src/libknot/internal/namedb/namedb_trie.c